Voltage-controlled oscillator with LC resonant circuit

ABSTRACT

A voltage-controlled oscillator device with an LC-resonant circuit, in particular for implementing integrated voltage-controlled oscillators for the lower GHz range, is disclosed. The device achieves continuous frequency tunability in a wide range in particular with a low level of phase noise and phase jitter. In the voltage-controlled oscillator, a second inductor can be periodically switched in parallel and/or in series with at least one first inductor of the LC-resonant circuit by way of a switching means actuated with the oscillator frequency. A control input of the switching means is connected to a variable dc voltage. In that respect the relationship of the duration of the conducting state and the duration of the non-conducting state of the switching means is variable within an oscillation period of the oscillator in dependence on the value of the control voltage. In accordance with the relationship of the duration of the conducting state and the duration of the non-conducting state of the switching means within an oscillation period of the oscillator the time-averaged effective inductance is variable in dependence on the value of the control voltage.

The invention concerns a voltage-controlled oscillator with anLC-resonant circuit, in particular for implementing integratedvoltage-controlled oscillators for the lower GHz range.

BACKGROUND OF THE ART

Integrated circuits involve using voltage-controlled oscillators whichare mostly in the form of ring oscillators or LC-oscillators. Ringoscillators are distinguished by a high degree of frequency tunability.That advantage however is impaired by a strong phase noise and a severephase jitter. In the case of LC-oscillators frequency tunability ispredominantly implemented by means of variable capacitors, for examplecapacitor diodes. Those oscillators admittedly involve a lower level ofphase noise and a lesser degree of phase jitter, but frequencytunability is in most cases seriously reduced.

Japanese patent application 093 215 38 A describes a voltage-controlledLC-oscillator circuit in which a part of the inductance isshort-circuited by means of a switching transistor for given periods oftime, whereby the inductive component is reduced at times in such a waythat alternate operation of the frequency in two frequency bands ispossible.

Apart from the switching operation which is substantially slower thanthe period duration in the desired frequency range, such an arrangementdoes not permit continuous tuning of the frequency in a wide frequencyrange.

A similar principle is also described in: A. Kral et al“RF-CMOS-Oscillators with Switched Tuning”, Custom Integrated CircuitsConference (CICC'98), pp. 555-558. In the case of a fully integratedCMOS-oscillator for a frequency range of between 1 and 2 GHz a tuningrange of about 26% is achieved by switching between a plurality ofdiscrete inductance values.

Besides the use of switching elements which adversely influence phasenoise and phase jitter, that arrangement is seen to suffer from thedisadvantage that, in spite of the circuit being of a high degree ofcomplexity, it was only possible to achieve a relatively limitedfrequency tuning range. In addition the switching of discrete inductancevalues means that it is only possible to achieve quasi-continuousfrequency tuning which has to be supplemented by capacitive tuning.

In integrated radio systems the oscillator must enjoy a relatively greattuning range in order to compensate for technology and temperaturefluctuations and to cover the receiving and transmitting bandrespectively.

With operating voltages in modern technologies becoming smaller andsmaller the available voltage range for the control voltage of thevoltage-controlled oscillator (VCO) is becoming progressively smaller.That means that the necessary sensitivity of the oscillation frequencyof the oscillator in relation to control voltage variations increases.The consequence of this is that, upon integration of the VCO into aphase-locked loop (PLL) the noise of the control voltage causes severephase noise. That problem is becoming more acute with down-scaling ofthe technology, which goes hand-in-hand with the reduction in the supplyvoltage.

Therefore the object of the invention is to propose a voltage-controlledoscillator with an LC-resonant circuit, with which the disadvantages ofthe state of the art are overcome and with which continuous frequencytunability in a wide range can be achieved in particular with a lowlevel of phase noise and a low level of phase jitter.

SUMMARY OF THE INVENTION

In accordance with the invention that object is attained in that, in avoltage-controlled oscillator with an LC-resonant circuit there can beperiodically switched in parallel and/or in series with at least oneinductor a further inductor by way of a switching means actuated withthe oscillator frequency and that a control input of the switching meansis connected to a variable dc voltage. Advantageously a further inductorcan be periodically switched in parallel and/or series with a pluralityof inductors by way of a respective controllable switching means. Thecontrollable switching means are periodically in a conducting state andthen a non-conducting state. They are controllable by a variable controlvoltage. In that respect the relationship of the duration of theconducting state and the duration of the non-conducting state of theswitching means is variable within an oscillation period of theoscillator in dependence on the value of the control voltage. Inaccordance with the relationship of the duration of the conducting stateand the duration of the non-conducting state of the switching meanswithin an oscillation period of the oscillator the time-averagedeffective inductance is variable in dependence on the value of thecontrol voltage. The controllable switching means are advantageouslyswitching transistors and in particular MOSFETs whose gate terminals areconnected to the input of the control voltage and whose source terminalsare connected to parts of the circuit arrangement carrying theoscillator frequency. Advantageously the oscillator is constructed usinga CMOS or bipolar technology and can preferably be used in frequencysynthesizers for wide-band systems and for multi-band uses and for clockgeneration and clock recovery in high-speed circuits such as for examplemicroprocessors and memories.

The teaching of the invention involves replacing the coils in a resonantcircuit by pairs of coils which are connected in parallel and/or inseries and of which a respective one of the coils is connected to aswitch which is periodically opened and closed. That means that in eachcase only one coil or the parallel and/or series connection of bothcoils is operative. The period of time during which the switch is closedwithin an oscillation period is controlled by a control voltage. Thetime-averaged effective inductance can thus be altered in a wide range.This results in the desired continuous frequency tunability.

BRIEF DESCRIPTION OF THE DRAWINGS

the features of the invention, besides being set forth in the claims,are also to be found in the description and the drawings, in whichrespect the individual features each on their own or in pluralities inthe form of sub-combinations represent patentable configurations inrespect of which protection is claimed herein. Embodiments by way ofexample of the invention are described in greater detail hereinafter. Inthe accompanying drawings:

FIG. 1 shows a voltage-controlled oscillator according to the invention,

FIG. 2 shows a diagram of the oscillator frequency as a function of thecontrol voltage,

FIG. 3 shows a further embodiment of the oscillator according to theinvention,

FIG. 4 shows a voltage-controlled oscillator according to the invention,

FIG. 5 shows a further embodiment by way of example of the oscillatoraccording to the invention,

FIG. 6 shows a combination circuit of a VCO with a PLL, and

FIG. 7 shows a combination circuit of a VCO with two PLLs.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT EXAMPLE 1

FIG. 1 shows an LC-oscillator according to the invention with twocooperating semiconductor switches and a capacitor C. The inductors L₁are arranged in two branches. Associated with each of the two inductorsL₁ is a respective further inductor L₂ which can be switched in parallelwith the first inductors L₁ by way of a respective switching meansS_(v). The gate terminals G of the switching means S_(v), which are inthe form of MOSFETs are connected to an input V_(con) for a controlvoltage U_(con) while the source terminals S are connected to the outputof the oscillator, which carries the oscillator frequency.

FIG. 2 shows a diagram in respect of the oscillator frequency in GHz asa function of the control voltage U_(con).

The mode of operation of the oscillator according to the invention is asfollows: the two switching means S_(v) in this embodiment being twoMOSFETs, are opened at a low control voltage U_(con) during the majorpart of an oscillation period of the oscillator. That state occurs aslong as the gate-source voltage does not exceed the switching point ofthe switching means S_(v). During the duration of the non-conductingstate of the switching means S_(v) only the first inductors L₁ areeffective. For a small part of the oscillation period the gate-sourcevoltage exceeds the switching point of the switching means S_(v). Forthe duration of the now conducting state of the switching means S_(v)the further inductors L₂ are connected in parallel with the firstinductors L₁ whereby the total value of the effective inductance reducesas a function of time. In accordance with the relationship of the longerduration of the non-conducting state of the switching means S_(v) to theshorter duration of the conducting state thereof, there is a relativelygreat, time-averaged effective inductance. The oscillator frequencyresulting therefrom is correspondingly low.

With an increased control voltage U_(con) the two switching means S_(v)are opened only during a smaller part of the oscillation period and areclosed during the major part thereof. In accordance with therelationship of the shorter duration of the non-conducting state of theswitching means S_(v) to the longer duration of the conducting statethereof there is therefore a relatively low, time-averaged effectiveinductance. The oscillator frequency resulting therefrom iscorrespondingly high.

As a special case it will be assumed that the inductance and quality ofthe pairs of coils L₁ and L₂ are identical and involve the samemagnitudes L1 and Q1. For the situation involving ideal switching meansS_(v) the following then apply in regard to the inductance L and thequality Q of the overall arrangement comprising L₁, L₂ and the switch:

-   -   L=L1, Q=Q1 when the switching means S_(v) are opened, and    -   L=L1/2, Q=Q1 when the switching means S_(v) are closed.

Closure of the switching means S_(v) therefore causes halving of theinductance which is crucial in terms of the oscillator frequency. Thequality of the pairs of coils L₁ and L₂ is equal to the quality of theindividual coil. If it is considered that the following approximatelyapplies for the oscillator frequency:f _(o)1=/√{square root over (L)},the following relationship is found for the lower limit frequencyf_(o),min and for the upper limit frequency f_(o),max for the frequencytuning range:f _(o),max=√{square root over (2)}·f _(o),min

The following similarly applies for the general case of coils which arenot necessarily the same:f _(o),max=√{square root over ((1+L1/L2))}·fo,min

Thus the frequency tuning range can still be further increased by thechoice of a greater ratio of L1/L2.

FIG. 2 shows the simulated frequency tuning range in the form of adiagram showing the oscillator frequency f_(o) as a function of thecontrol voltage U_(con) for L1/L2=2. In this embodiment the frequencytuning range is about 1.25 GHz, that is to say more than an octave.

The oscillator according to the invention can be implemented in a fullyintegrated configuration both using CMOS technology and also bipolartechnology. It can advantageously be used in frequency synthesizers forwide-band systems and for multi-band uses and for clock production andclock recovery in high-speed circuits such as microprocessors andmemories.

EXAMPLE 2

As a further embodiment by way of example FIG. 3 shows a circuitarrangement of the oscillator according to the invention with tworespective first inductors L₁, L₃, in relation to each of which afurther respective inductor L₂ can be connected in parallel.

The frequency tuning range can be increased by the use of more than twoinductors L₁, L₂, as demonstrated in FIG. 3.

EXAMPLE 3

FIG. 4 shows an LC-oscillator according to the invention with twoco-operating semiconductor switches and a capacitor C. The inductors L₁are arranged in two branches. Associated with each of the two inductorsL₁ is a respective further inductor L₂ which can be connected in serieswith the first inductors L₁ by a respective switching means S_(v). Thegate terminals G of the switching means S_(v) which are in the form ofMOSFETs are connected to an input V_(con) for a control voltage U_(con)while the source terminals S are connected to the output of theoscillator, which carries the oscillator frequency.

When the switching means S_(v) is closed the total inductance is of alower value than when the switching means S_(v) is opened. The switchingmeans S_(v) is modulated at the oscillation frequency.

The mode of operation of the oscillator according to the invention is asfollows: the two switching means S_(v), in this embodiment being twoMOSFETs, are opened with a low voltage U_(con) at the input V_(con)during the major part of an oscillation period of the oscillator. Thatstate occurs as long as the gate-source voltage does not exceed theswitching point of the switching means S_(v). During the duration of thenon-conducting state of the switching means S_(v) the further inductorsL₂ are effective, in relation to the first inductors L₁, whereby thetotal value of the effective inductance is increased. For a small partof the oscillation period the gate-source voltage exceeds the switchingpoint of the switching means S_(v). Only the first inductors L₁ areeffective for the duration of the now conducting state of the switchingmeans S_(v). In accordance with the relationship of the longer durationof the non-conducting state of the switching means S_(v) to the shorterduration of the conducting state thereof, there is a relatively high,time-averaged effective inductance. The oscillator frequency resultingtherefrom is correspondingly low.

With an increased control voltage U_(con) the two switching means S_(v)are only opened during a smaller part of the oscillation period andclosed during the greater part thereof. In accordance with therelationship of the shorter duration of the non-conducting state of theswitching means S_(v) to the longer duration of the conducting statethereof, there is a relatively low, time-averaged effective inductance.The oscillator frequency resulting therefrom is correspondingly higherthan with a lower control voltage U_(con).

EXAMPLE 4

FIG. 5 shows a combination of inductive and capacitive tuning. Besidesinductive tuning, capacitive tuning is also possible.

Inductive tuning is based on the principle described in the precedingembodiments. In this embodiment the inductors L₁ and L₂ are connected inparallel. The two switching means S_(v) are opened at a low controlvoltage U_(con) at the input V_(con) during the major part of anoscillation period of the oscillator. That state occurs as long as thegate-source voltage does not exceed the switching point of the switchingmeans S_(v). During the duration of the non-conducting state of theswitching means S_(v) only the first inductors L1 are effective. For asmall part of the oscillation period the gate-source voltage exceeds theswitching point of the switching means S_(v). For the duration of thenow conducting state of the switching means S_(v) the further inductorsL2 are connected in parallel with the first inductors L1 whereby thetotal value of the effective inductance decreases as a function of time.In accordance with the relationship of the longer duration of thenon-conducting state of the switching means S_(v) to the shorterduration of the conducting state thereof there is a relatively hightime-averaged effective inductance. The oscillator frequency resultingtherefrom is correspondingly low.

With an increased control voltage U_(con) the two switching means S_(v)are opened only during a smaller part of the oscillation period and areclosed during the major part thereof. In accordance with therelationship of the shorter duration of the non-conducting state of theswitching means S_(v) to the longer duration of the conducting statethereof there is a relatively low, time-averaged effective inductance.The oscillator frequency resulting therefrom is correspondingly high.

To provide for capacitive tuning integrated in the resonant circuit is avariable capacitance which in this embodiment is embodied by means oftwo p-MOSFETs M₁, M₂ in the form of variable capacitor diodes. Thecontrol input V_(con) permits tuning of the frequency on the basis ofthe principle described in the preceding embodiments while a tuningvoltage U_(tune) at the tuning input V_(tune) determines the oscillationfrequency by way of the time-averaged capacitance. It is now possible touse the control input V_(con) in order to compensate for technologyfluctuations while the tuning input V_(tune) is used for fine tuning bymeans of a phase-locked loop PLL, as shown in FIG. 6. In this case, thetuning input V_(tune) of the VCO is connected to the output of thephase-locked loop PLL and the oscillator output of thevoltage-controlled oscillator VCO is connected to the input of thephase-locked loop PLL.

In this case it is possible to use a relatively low VCO-gainK=df_(o)/dU_(tune). In that way the effect of noise within thephase-locked loop PLL on the phase noise of the voltage-controlledoscillator VCO is minimized. The noise of the inductive control voltageat the control input V_(con) can be blocked by means of a largecapacitance.

EXAMPLE 5

A modified variant is illustrated in FIG. 7. There, the oscillatoroutput is connected to the inputs of two phase-locked loops PLL1 andPLL2. The tuning input V_(tune) of the voltage-controlled oscillator VCOis connected to the output of the phase-locked loop PLL1 while thecontrol input V_(con) of the voltage-controlled oscillator VCO isconnected to the output of the phase-locked loop PLL2.

The phase-locked loop PLL2 serves to compensate for technology andtemperature fluctuations while the phase-locked loop PLL1 serves forfine tuning of the oscillation frequency.

This method is particularly suitable for a modulation method which isreferred to as frequency hopping. This is a special code divisionmultiple access method (CDMA) in which the transmitting and receivingfrequency are altered in respect of time in accordance with apredetermined code. This can be implemented by means of the phase-lockedloop PLL1 while the very slow phase-locked loop PLL2 provides for coarsesetting of the frequency.

A use of the invention is the “Bluetooth” standard for wirelesscommunication over short distances. The frequency hopping method is usedthere. The demands in terms of phase noise are not too high there, whichmakes an integrated CMOS-solution a possibility.

A voltage-controlled oscillator with an LC-resonant circuit wasdescribed in the foregoing description by means of specific embodiments.It should be noted however that the present invention is not limited tothe details of the described in the specific embodiments asmodifications and alterations are claimed within the scope of theclaims.

1. A voltage-controlled oscillator oscillating at a controllableoscillator frequency comprising: an LC-resonant circuit with at leastone first inductor; at least one controllable switching device,connected to said LC-resonant circuit to periodically take on aconducting and a non-conducting state at the oscillator frequency; andat least one second inductor which can be periodically switched inparallel or in series connection with said at least one first inductorof the LC-resonant circuit by way of the at least one controllableswitching device actuated at the oscillator frequency; wherein the atleast one controllable switching device has a control input forcontrolling, by means of a control voltage, a portion of an oscillationperiod of the LC-resonant circuit during which portion the secondinductor is connected to said LC-resonant circuit.
 2. Thevoltage-controlled oscillator of claim 1, wherein: the at least onesecond inductor is arranged to be periodically switchably connected tothe LC-resonant circuit in parallel to said at least one said firstinductor.
 3. The voltage-controlled oscillator of claim 2, wherein: thetime-averaged effective inductance varies, depending on the controlvoltage according to the relationship of the duration of the conductingstate and the duration of the non-conducting state of the switchingmeans within an oscillation period of the oscillator.
 4. Thevoltage-controlled oscillator of claim 2, wherein: the at least onecontrollable switching device comprises switching transistors.
 5. Thevoltage-controlled oscillator of claim 4, wherein: the switchingtransistors are MOSFETs.
 6. The voltage-controlled oscillator of claim5, wherein: the MOSFETs have gate terminals that are connected to thecontrol input of the control voltage.
 7. The voltage-controlledoscillator of claim 6, wherein: the MOSFETs have source terminals thatare connected to parts of a circuit arrangement carrying the oscillatorfrequency.
 8. The voltage-controlled oscillator of claim 7, wherein: thevoltage controlled oscillator is of a CMOS or bipolar technology.
 9. Thevoltage-controlled oscillator of claim 1, wherein: the relationship ofthe duration of the conducting state and the duration of thenon-conducting state of the at least one controllable switching devicewithin an oscillation period of the oscillator varies, depending on thecontrol voltage.
 10. The voltage-controlled oscillator of claim 1,wherein: the at least one controllable switching device comprisesswitching transistors.
 11. The voltage-controlled oscillator of claim10, wherein: the switching transistors are MOSFETs.
 12. Thevoltage-controlled oscillator of claim 11, wherein: the MOSFETs havegate terminals that arc connected to the control input of the controlvoltage.
 13. The voltage-controlled oscillator of claim 12, wherein: theMOSFETs have gate terminals that are connected to parts of a circuitarrangement carrying the oscillator frequency.
 14. Thevoltage-controlled oscillator of claim 1, wherein: the oscillator is ofa CMOS or bipolar technology.
 15. The voltage-controlled oscillator ofclaim 1, wherein: the oscillator is used in frequency synthesizers forwide-band systems and for multi-band uses and for clock production andclock recovery in high-speed circuits such as for examplemicroprocessors and memories.
 16. The voltage-controlled oscillator ofclaim 1, wherein: a voltage-controlled capacitance is integrated in theoscillator, which is connected to a tuning voltage by way of a furthercontrol input, a tuning input.
 17. The voltage-controlled oscillator ofclaim 16, wherein: the voltage-controlled capacitance is embodied bymeans of at least one variable capacitor diode, wherein an effectivecapacitance depends on the tuning voltage at the tuning input.
 18. Thevoltage-controlled oscillator of claim 16, wherein: the tuning input ofthe oscillator is connected to an output of a phase-locked loop and theoutput of the voltage-controlled oscillator is connected to an input ofthe phase-locked loop.
 19. The voltage-controlled oscillator of claim16, wherein: the tuning input of the voltage-controlled oscillator isconnected to the output of a phase-locked loop and the control input ofthe voltage-controlled oscillator is connected to an output of a furtherphase-locked loop.
 20. The voltage-controlled oscillator of claim 16,wherein: the voltage-controlled capacitance is embodied by means of atleast one variable capacitor diode, wherein an effective capacitancedepends on the tuning voltage at the tuning input.
 21. Thevoltage-controlled oscillator of claim 20, wherein: the tuning input ofthe voltage controlled oscillator is connected to an output of aphase-locked loop and the output of the voltage controlled oscillator isconnected to an input of the phase-locked loop.
 22. Thevoltage-controlled oscillator of claim 1, wherein: the noise of thecontrol voltage at the control input is blocked out by means of a highcapacitance between the control input and ground.
 23. Thevoltage-controlled oscillator of claim 1, wherein: the at least oneinductor is arranged to be periodically switchably connected to theLC-resonant circuit in series with one said at least one inductor. 24.The voltage-controlled oscillator of claim 23, wherein: thetime-averaged effective inductance varies, depending on the controlvoltage according to the relationship of the duration of the conductingstate and the duration of the non-conducting state of the switchingmeans within an oscillation period of the voltage controlled oscillator.25. The voltage-controlled oscillator of claim 23, wherein: the at leastone controllable switching device comprises switching transistors. 26.The voltage-controlled oscillator of claim 25, wherein: the switchingtransistors are MOSFETs.
 27. The voltage-controlled oscillator of claim26, wherein: the MOSFETs have gate terminals that are connected to thecontrol input of the control voltage.
 28. The voltage-controlledoscillator of claim 27, wherein: the MOSPETs have source terminals thatare connected to parts of a circuit arrangement carrying the oscillatorfrequency.
 29. The voltage-controlled oscillator of claim 28, wherein:the voltage controlled oscillator is of a CMOS or bipolar technology.30. The voltage-controlled oscillator of claim 1, wherein: theLC-resonant circuit has at least two first inductors; and the at leastone inductor is arranged to be periodically switchably connected to theLC-resonant circuit, in series with a first of the at least two firstinductors and in parallel with a second of the at least two firstinductors.
 31. The voltage-controlled oscillator of claim 30, wherein:the at least one controllable switching device comprises switchingtransistors.
 32. The voltage-controlled oscillator of claim 31, wherein:the switching transistors are MOSFETs.
 33. The voltage-controlledoscillator of claim 32, wherein: the MOSFETs have gate terminals thatare connected to the control input of the control voltage.
 34. Thevoltage-controlled oscillator of claim 33, wherein: the MOSFETs havesource terminals that are connected to parts of a circuit arrangementcarrying the oscillator frequency.
 35. The voltage-controlled oscillatorof claim 34, wherein: the voltage controlled oscillator is of a CMOS orbipolar technology.